I. Field of Invention
This invention relates to the digital servo control or servo mechanism field, and more particularly, to an improved digital servo control for operating a servo motor having a resolver feedback element for controlling accurately the position of a shaft, such as in a servo motor, and to use this angular shaft output to drive a ball screw mechanism.
II. Description of the Prior Art
The essential features of a servo mechanism are the servo motor itself, the drive amplifier, and the servo amplifier which can be either an analog or a digital type unit. The servo amplifier will feed the drive amplifier which is nothing more than a high current output and voltage source for operating the motor usually at varying DC voltages. The system will normally have a tachometer feedback which will continuously provide a velocity signal to the drive amplifier. This signal assists to control the rotary position and speed of the output shaft. In addition, a position feedback device such as a resolver or an encoder are sometimes used which feed directly back to the servo amplifier and provide signal information as to the actual position which the motor shaft has attained. As noted earlier, the different types of systems can be analog or digital. The analog system relies on the actual value of voltages and currents to control the output of the servo amplifier and the drive amplifier in order to control the position, speed, and acceleration of the servo motor. Most recently, digital techniques have been adapted in order to get a greater sensitivity of control over the servo mechanisms. One such device is shown in U.S. Pat. No. 3,340,447 to MacDonald, titled Digital Servo Mechanism. This patent details the use of digital signals to position and output shaft which has a feedback scheme and other circuit means for instructing the drive amplifier that it has attained the correct position or speed and provides continuous signals for operating a servo motor.
Currently available on the market is a servo amplifier built by Hyperloop Incorporated of Bridgeview, Ill. This device is designed to work with a servo motor drive system having a tachometer and resolver feedback network. The resolver tells the control system or the servo amplifier the exact position of the output shaft. This system called the MEGA-HIGHSTEP has the unique capability over other similar devices on the market in that it is capable of moving at high rates of speed and still provide fine positioning accuracy. For example, this device can handle data at the rate of 1,000,000 bits per second or 1 Megahertz (MHz). Whereas, most other devices on the market are in the order of 100,000 to 200,000 bits per second or 0.1 to 0.2 MHz. This device offered an improvement of from 5 to 10 times the speed and accuracy combination of previous type devices. However, the resolver feedback mechanism in the system has a large error associated with it which resulted in excessive positioning errors when used in conjunction with a slide system in a machine tool. In such a machine tool system, the output shaft of the servo motor feeds a ball screw and thereby positions a machining table or machining element. The error occurring in the system is caused from the basic design of the resolver and the resolver manufacturer cannot provide any better resolution than the current technology allows at this time. In addition to these errors, the feedback package connecting to the servo motor usually has gears coupling the resolver to the motor shaft. These gears, even though they are of high precision and quality, introduced additional errors into the positioning system. Obviously, because the gears are subject to wear, continuous use of this type of system will only increase the error of the overall system as time goes on. Unfortunately, this state of the art technology left the inventors with a positioning system far too inaccurate to be useful in a high precision machine tool application involving a grinding operation.
This invention is a new and novel method to achieve more accurately the angular position of the output shaft of a servo motor and thereby increase the accuracy of the position of a ball screw device used for the linear translation or position of a slide on a machine tool table. This new device is a rotary position transducer commercially called an INDUCTOSYN.RTM. device and is manufactured by Farrand Controls of Valhalla, N.Y. This is connected to the output shaft of the motor directly and has the capability of providing a very high resolution or feedback signal which has not been used before with a servo amplifier in such a system application. Since the transducer is connected directly to the motor shaft, no gearing wear or other inaccuracies are introduced in the system.
An object of this invention was to utilize a quadrature phasing network to develop two specific signals to feed the rotary position transducer. Another object of this invention was to shape and condition the pulses coming from the quadrature phasing network which ultimately fed a balance amplifier for maintaining the linearity of the signal into the Inductosyn or rotary position transducer.
Another object of this invention is to pick off the feedback signal and amplify that signal through a preamplifier and filter and feed it into the servo amplifier. A further object of this invention is to utilize the angular position of the output shaft in conjunction with a ball screw mechanism for operation with a machine tool element. An object of this invention is to produce a sine and cosine signal for energizing the rotary position transducer coils. An object of this invention is to determine the phase shift between the rotor and stator of the rotary position transducer and detect and amplify that phase difference through a pre-amp and band pass filter into the servo amplifier. Another object of this invention is to convert the digital feedback signal into a sinusoidal signal which will feed the servo amplifier.